chore: Harmonize imports of sqlalchemy module, use sa where applicable

It follows a convention to import SQLAlchemy like
`import sqlalchemy as sa`. In this spirit, all references, even simple
ones like symbols to SQLAlchemy base types like `TEXT`, or `BIGINT`,
will be referenced by `sa.TEXT`, `sa.BIGINT`, etc., so it is easy to
tell them apart when harmonizing type definitions coming from SA's
built-in dialects vs. type definitions coming from 3rd-party dialects.